I had a similar issue with something I was trying to replicate that replaced the Dex armor bonus with the minion's proficiency bonus. This was a pretty 'hacky' way to make it work, but the numbers add up and it's only a couple of lines in an eval script. I had this run in the final phase. You could target your hero with the first two lines, and it would accomplish removing the dex bonus, but I'm fully aware this might not be the best solution to your issue.
Code:
minion[Familiar].childfound[ArmorClass].field[tACBase].value = 18
minion[Familiar].childfound[ArmorClass].field[tACDexMod].value = 0
var profnum as number
profnum = minion[Familiar].childfound[ProfBonus].field[tProfBonus].value
minion[Familiar].childfound[ArmorClass].field[tACNatural].value = profnum